Variable area exhaust nozzles for gas turbine engines are used in a variety of aircraft applications to optimize engine thrust in response to varying temperature, pressure, and other environmental and operational conditions. Such nozzles include a primary flow path, typically exhausting the working fluid axially, which is equipped with a flap or other means for selectably varying the nozzle outlet area.
During landing and/or other maneuvers, it is desirable to divert the working fluid from the axial direction to induce a vectored thrust for maneuvering or slowing the aircraft. One such thrust vectoring arrangement provides an alternate working fluid discharge path within the nozzle structure, including means for admitting the working fluid into such alternate discharge path and closing off or otherwise substantially restricting the primary axial flow discharge path. An example of such arrangement is shown in U.S. Pat. No. 4,605,169, wherein a pivoting convergent flap moves into the axially flowing working fluid for blocking off a portion of the axial path flow area as required to provide optimum engine thrust. This same flap blocks off the alternate discharge path over a normal range of flap pivoting motion, unblocking the alternate flow path when pivoted into the primary flow path beyond a certain point, thereby admitting working fluid into the alternate path for generating reverse thrust.
Another arrangement is shown in commonly assigned, pending U.S. patent application Ser. No. 030,794, titled "Method and Linkage for Positioning a Convergent Flap and Coaxial Arc Valve", filed on Mar. 27, 1987, now U.S. Pat. No. 4,767,055, which discloses a convergent flap pivotable into the primary axial working fluid discharge path and including a semicylindrical arc valve normally blocking the alternate working fluid discharge path. The arc valve and convergent flap are joined by a mechanical linkage which opens the arc valve in response to movement of the convergent flap into the primary exhaust path resulting in a primary path flow area less than a preset minimum area. Both the linkage shown in application Ser. No. 030,794 and the arrangement shown in U.S. Pat. No. 4,605,169 are operable to divert the working fluid flow between the primary axial flow discharge path and the alternate thrust vectoring flow discharge path responsive to the movement of the convergent flap only. Thus a single actuator provides both axial thrust area variation and diversion of the working fluid into the thrust vectoring flow path.
The saving in weight realized by the use of a single actuator is offset in the prior art arrangements by the operational inflexibility caused by linking the movement of the convergent flap to the opening of the alternate working fluid discharge path. In each case, the collective nozzle flow discharge area must be reduced to a preset minimum area before the alternate working fluid discharge path begins to open. The selection of the preset minimum area is responsive to two, possibly conflicting, considerations: (1) the minimum nozzle area necessary to provide optimum engine thrust over all engine operating conditions, and (2) the minimum acceptable nozzle outlet area for engine stability over all operating conditions.
For those engine-nozzle configurations wherein the minimum nozzle area based on stability considerations is greater than the minimum nozzle area necessary for optimum thrust, the opening of the alternate working fluid discharge path must be scheduled so as to insure stable engine operation at all points in the engine operating envelope. Thus the nozzle arrangement is unable to achieve optimum thrust under certain conditions due to an over area condition in the primary axial working fluid discharge path.
One solution to this over area condition is to provide a separate actuator for the alternate discharge path regulating means, thereby allowing the nozzle control system to schedule the opening of the alternate path based on current engine and environmental conditions. As noted above, such additional actuators and compexity is undesirable from weight, cost, and operational considerations.